Sydney woman 'panicked' before allegedly plunging kitchen knife into her fiance

Sydney woman Marcela Castaneda has told a NSW Supreme Court jury her fiance choked her so hard she could not scream in the moments before she fatally plunged a kitchen knife into his chest.

The jury was this week directed to find Ms Castaneda not guilty of murdering long-time partner Gregory Peck in February 2013 in their western Sydney home, but she remains on trial for manslaughter.

The 33-year-old sobbed and shook in the witness box on Wednesday as she told of her "panic" before the killing.

Ms Castaneda testified she called their wedding off that night, February 22 2013, and told Mr Peck: "I don't want you here anymore."

"All of a sudden I could feel his hands around me," she said.

"I could feel pressure around my neck and I was struggling to breathe, and I could feel him on top of me ... I couldn't scream."

She said her fear intensified when Mr Peck tried to follow her into the kitchen.

"That scared me, because he'd never come after me before. So then when I saw the knife, I just grabbed it," she said.

Ms Castaneda gave evidence she tried to lead Mr Peck from their home at knife-point - but when he "lunged" at her she was terrified and tried to fend him off with the blade.

"I thought he was going to hit me," she said, as family members sitting in the public gallery wiped away tears.

"I was waiting for the impact, I was bracing ... I opened my eyes and looked up, and I saw he just stood there for a while, and then he turned around and he mumbled something under his breath."

The knife was still in his chest, she said.

It was then, Ms Castaneda says, that she realised Mr Peck was seriously hurt and called an ambulance.

But he could not be saved.

The trial continues.